1. Field of the Invention
The invention relates generally to sensors and more particularly to semiconductor electromechanical sensors that include a vibrating element.
2. Description of the Related Art
A variety of sensors have been built which are based on the principle that the resonant vibrational frequency of a vibratable element depends upon the stress applied to it. For example, sensors using a vibratable element have been built which can measure such diverse physical parameters as acceleration, viscosity, fluid flow rate and pressure.
Semiconductor electromechanical sensors employing vibratable elements offer numerous advantages such as small size, batch fabrication, relatively high values of Q and the availability of well-developed semiconductor processing technologies. Semiconductor sensors have been constructed in which the vibratable element, for example, comprises a cantilever beam or a microbridge.
While earlier semiconductor sensors generally have been successful, there have been shortcomings with their use. In particular, for example, mismatches in the materials present in such prior sensors can lead to a degradation in sensor performance. For example, sensors employing a cantilever beam or a microbridge often suffer from mismatches in coefficients of thermal expansion as between the beam or microbridge and the semiconductor substrate of the sensor. Additionally, for example, the Young's Modulus of the beam or microbridge of earlier sensors often is not well matched with the Young's Modulus of the substrate. Furthermore, for example, residual strains often are present in such sensors as a result of the variety of material types employed in the sensors.
Moreover, earlier semiconductor sensors, of a type that employ an electrostatic drive apparatus to stimulate vibration of a vibratable element or that employ a capacitive pick-up apparatus to measure the resonant frequency of the vibratable element, often use an additional semiconductor wafer to embody the drive apparatus or the pick-up apparatus. Often, avoiding the use of such an additional wafer can be desirable.
Thus, there has been a need for a semiconductor sensor that substantially does not suffer from mismatches in the materials employed in the sensor. In particular, there is a need for such a sensor that substantially does not suffer from mismatches in coefficients of thermal expansion or Young's Modulus and that does not suffer from internal residual strains. There also exists a need for such a sensor in which electrostatic drive or capacitive pick-up can be readily employed without the need for an additional semiconductor wafer. The present invention meets these needs.
Furthermore, both the electrical and mechanical properties of semiconductors are temperature dependent. Consequently, for example, the resonant frequency of a cantilever beam or a microbridge can be temperature dependent. Thus, there exists a need for a semiconductor sensor in which temperature can be closely monitored. The present invention also meets this need.